First I would say that E is more specific and points directly to the flaw. I have seen something similar have to do with the weight of a baby. It says that the average baby gains a certain percentage of weight and so if the baby in question weighs less after three months (or something) then the baby has not gained weight quickly enough. This is playing off of the fact that the baby could simply have started smaller and may in fact have gained weight very quickly.

As to choice B, this is something the would be rare as an answer choice on the GMAT but it very frequent on the LSAT. If you want a handy way to test for "ambiguous word usage" you can do the following, use a synonym for the first use of the term (to be ambiguous the term must have been used at least twice - if only used once then not ambiguous). Then try to use the same synonym on the second use of the term.

For example, "The news media serves the public interest, so they should report only on things the public is interested in." Now if we choose a synonym for the first use of interest we might say public "good" so lets try to use that for the second usage of interest - it does not work as we see that here interest means more like "curiosity" so this is ambiguous word usage.

In this question we can use "normal" in place of average since this is what is meant and it works each time. "a [normal] main sequence star....its luminosity has increased much slower than [normal]. No ambiguity there - average means the same thing both times.

Sorry to get technical on that, as I said this will not come up often on the GMAT but it helps to expand our critical reasoning abilities!

whats the source ? didnt find it anywhere else on internet

Glad to see it is not on the Internet yet!

This is from the Veritas Prep Critical Reasoning 2 book.